1. Field of the Invention
This invention relates to a beam splitter suitable for splitting part of an imaging light beam to form object images on or near a plurality of rows of photosensors such as CCD sensors in which a number of independent photoelectric conversion elements are arranged.
In an in-focus point detecting apparatus wherein, as in a camera with a focus indicator or an AF camera, a light-receiving device such as a CCD sensor is disposed on or near the substantial image plane within the camera and the focus adjusted condition is detected by means of object image information obtained from the light-receiving device, this invention is particularly useful as a device for splitting an imaging light beam while maintaining the imaging performance of the imaging light beam so that the light-receiving device may obtain accurate information of the focus condition .
2. Description of the Prior Art
As a beam splitter employed when it is necessary to maintain the imaging performance of an imaging light beam after it is split, use has almost been made of an amplitude division type beam splitter comprising a glass substrate having a dielectric semi-transparent film deposited thereon by evaporation.
However, such a dielectric semi-transparent mirror has suffered from the problem that the color characteristic and polarization characteristic of the light beam after being split are aggravated. The applicant has previously proposed in Japanese Patent Application No. 98524/1980 a beam splitter which solves such problem and yet incorporates therein an area type beam splitter which can be suitably utilized in an image processing system using image detecting elements such as CCDs arranged at a predetermined pitch.
FIG. 1 of the accompanying drawings in a model view for schematically illustrating the in-focus point detecting apparatus of a camera disclosed in Japanese Laid-open Patent Application No. 18652/1980 already filed by the applicant. During the in-focus, the image of an object, not shown, is formed on a predetermined image plane 5 by an imaging optical system 1. At this time, from the outputs from a sensor array 2 disposed immediately forwardly of the predetermined image plane, a sensor array 3 disposed on the imaging plane and a sensor array 4 disposed immediately rearwardly of the image plane, there are obtained time-serial electrical signals of illumination distributions corresponding to a blurred image, a sharp image and a blurred image as is schematically shown in FIGS. 1 (a), (b) and (c), respectively. Accordingly, the in-focus state, the front focus state or the rear focus state may be detected on the basis of the information obtained from these three sensor arrays. This in-focus point detecting method is described in detail in the aforementioned Japanese Laid-open Patent Application No. 18652/1980. As is described in that patent application, the sensor arrays need not always be three, but in-focus point detection can also be accomplished by two sensor arrays disposed immediately forwardly and rearwardly of the image plane.
FIG. 2 of the accompanying drawings is a schematic view showing the interior of a camera, and FIG. 3 of the accompanying drawings is an enlarged view of a portion thereof. The camera of FIGS. 2 and 3 is sectioned along a plane containing the optical system to enable the interior of the camera to be easily seen.
In FIGS. 2 and 3, an imaging light beam 6 from an imaging optical system (not shown) enters a quick return mirror 12 and part of it passes through the semi-transparent mirror on the surface thereof while the remainder of the light beam is reflected toward a finder system. The back of the quick return mirror is provided with a light-intercepting material for intercepting any other light than the light travelling to a mirror 13, and the light beam having entered the mirror 13 is reflected to provide an imaging light beam 14 which enters an a minute beam splitter for in-focus point detecting system provided below the mirror 13.
In FIG. 3, the imaging light beam 14 which has passed through the beam splitter 12, enters an area type beam splitter 9.sub.1 and is further split thereby, and the reflected light therefrom is split by an area type beam splitter 9.sub.2 and the light passed through the area type beam splitter 9.sub.2 is reflected by an ordinary mirror 9.sub.3 and thus, there are provided three imaging light beams 10.sub.1, 10.sub.2 and 10.sub.3, which enter three CCD line sensors 11.sub.1, 11.sub.2 and 11.sub.3, respectively.
FIG. 4 of the accompanying drawings is an enlarged view of a minute beam splitter portion 8. This minute beam splitter portion 8 is formed of a transparent substance such as LASF.sub.016 having a refractive index nd=1.772 or BK7 having a refractive index nd=1.516, and the area type beam splitters 9.sub.1, 9.sub.2 and the ordinary mirror 9.sub.3 are provided in the minute beam splitter portion 8.
The transparent material used for the minute beam splitter portion also has the function of adjusting the optical path length difference (amount of defocus) between the imaging light beams entering sensors 11.sub.1-3, by the refractive index thereof.
As the amount of blur of the image for each sensor to detect the in-focus point, although it depends on the logic of the focus detecting system and the type of the interchangeable lens used, it is desirable that the optical path length differences between the imaging light beams 10.sub.1 and 10.sub.2 and between the imaging light beams 10.sub.2 and 10.sub.3 be 0.2-1.0 mm as converted into the optical path length difference in the air (actual optical path length/refractive index of the medium) and correspondingly thereto, the spacing d between the line sensors is set to the order of 0.5-2.0 mm.
Also, the spacing l from the position whereat the imaging light beams 10.sub.1, 10.sub.2 and 10.sub.3 enter the surfaces of the respective beam splitters 9.sub.1, 9.sub.2 and 9.sub.3 to the surface of each sensor is 1-2 mm as converted into the optical path length in the air, and the pitch P between the segments of CCD line sensors 11.sub.1, 11.sub.2 and 11.sub.3 is about 30 .mu.m.
In the minute beam splitter portion of such construction, as shown in FIG. 4, area type beam splitters (random dot mirrors) 9.sub.1 and 9.sub.2 each having small circular transmitting portions arranged at random are disposed at an angle of 45.degree. with respect to the sensor surface. The random dot mirror 9.sub.1 is designed such that the quantity of light T transmitted therethrough is T=about 33% .+-.about 2% as calculated simply in the area ratio between the light transmitting portion and the light reflecting portion when the mirror surface is scanned in the direction of sensor arrangement by an imaging light beam corresponding to F5.6-F8, and the random dot mirror 9.sub.2 is designed such that T=about 50% .+-.about 3%. Strictly, the area ratio is determined with the absorption characteristic of the reflecting film and the intensity distribution of the point image taken in account. The reason why the small circles are disposed at random is that the degradation of the intensity distribution of the point image will become great due to diffraction if they are disposed periodically, or to avoid a situation in which, where there are light beams passing through two area type mirrors 9.sub.1 and 9.sub.2 as shown in FIG. 4, moire is created between the patterns on the mirror surfaces and an error is caused in intensity ratio between the sensors or uniformity is lost when the sensor surface is illuminated by a uniform light beam. However, if the arrangement of the small circles is too random, non-uniformity will be caused in the illumination of the sensor surface and therefore, care must be taken of the balance of the arrangement.
It is desirable that the average size of the minimum diameter of the light transmitting portion or the light reflecting portion on the light splitting surface of the area type beam splitter indicated by small circles in FIG. 4 be greater than l/100 and smaller than l/10, where l is the air-converted optical path length from the beam splitter to the sensor surface. The reason is that if said average size is smaller than l/100, MTF of the split light will be remarkably reduced and if said average size is greater than l/10, the uniformity of the image will be aggravated when a lens of F5.6 or F8 is used and the light beam for imaging each point becomes thin. According to our experiment, the optimum value in the balancing of such value of MTF and the uniformity was about l/30 in the case of the previously described sensor.
The incident light beam 14 is split by the use of the area type beam splitters 9.sub.1 and 9.sub.2 and the respective split imaging light beams 10.sub.1, 10.sub.2 and 10.sub.3 are directed to CCD line sensors 11.sub.1, 11.sub.2 and 11.sub.3 and the amount of blur of the image is detected as shown in FIG. 1, whereby the focus adjusted condition can be known. In order to accurately know the proper focus adjusted condition, it is necessary that in each beam splitter, beam splitting be effected with the three imaging light beams 10.sub.1, 10.sub.2 and 10.sub.3 being substantially at the ratio of quantity of light of 1 : 1 : 1 so that there is no difference in color characteristic and polarization characteristic. The area type beam splitter is less expensive as compared with a dielectric semi-transparent mirror and can satisfy the above condition.
Where the beam splitter is adapted for use with a focus detecting apparatus, it is desirable that balance be kept with respect also to the MTF of the imaging optical system up to the respective sensors 11.sub.1 -11.sub.3. This is because, as described in the aforementioned Japanese Laid-open Patent Application No. 18652/1980, it is preferable for a focus detecting apparatus using the blur detecting method to detect the in-focus position by comparing the integrated values of the differences in output between adjacent segments with respect to the respective sensors.
Accordingly, when a camera is turned to an object of uniform brightness, no signal should be produced in the in-focus detecting apparatus of the blur detecting type as shown in FIG. 1, but when the pattern of the area type beam splitter is projected upon the sensor surface, some signal may be produced to effect a wrong focusing operation.